In recent years, demand for cold-rolled steel sheets having a high strength such as a tensile strength of 590 MPa or more has increased to comply with the trends toward automobile weight-reduction. Automobile-use cold-rolled steel sheets are painted and, prior to painting, a chemical conversion treatment such as phosphating is performed. The chemically conversion treatment to the cold-rolled steel sheet is one of the key processes for yielding corrosion resistance after painting.
Addition of Si effectively increases the strength of cold-rolled steel sheets. However, in steel sheets (high-strength cold-rolled steel sheets) containing Si, oxidation of Si occurs even in a reducing N2+H2 gas atmosphere that does not oxidize Fe (in other words, that reduces Fe oxides) during continuous annealing, and a thin film of a Si oxide (SiO2) is formed on the outermost surface of steel sheets. Since this Si oxide (SiO2) thin film inhibits the reaction for generating chemical conversion coatings during the chemical conversion treatment, micro regions in which no chemical conversion coatings are formed (hereinafter these regions are also referred to as “uncovered regions”) are generated and the chemical convertibility is degraded.
Patent Literature 1 describes a related art for improving the chemical convertibility of high-strength cold-rolled steel sheets, which is a method that includes controlling a steel sheet temperature to 350° C. to 650° C. in an oxidizing atmosphere to form an oxide film on a steel sheet surface, heating the steel sheet to a recrystallization temperature in a reducing atmosphere, and cooling the steel sheet.
Patent Literature 2 describes a method that includes forming an oxide film on a surface of a cold-rolled steel sheet in an iron-oxidizing atmosphere at a steel sheet temperature of 400° C. or higher, the cold-rolled steel sheet containing, in terms of mass %, 0.1% or more of Si and/or 1.0% or more of Mn, and then reducing the oxide film on the steel sheet surface in an iron-reducing atmosphere.
Patent Literature 3 describes a high-strength cold-rolled steel sheet in which oxides effective for improving chemical convertibility and other properties are contained in a crystal grain boundary and/or inside a crystal grain on a high-strength cold-rolled steel sheet surface layer containing 0.1 wt % or more and 3.0 wt % or less of Si. Patent Literature 4 describes a steel sheet having high phosphatability, in which, when a cross-section taken in a direction orthogonal to the steel sheet surface is observed with an electron microscope at a 50000× magnification or more and the ratio of the Si-containing oxides in a steel sheet surface length of 10 μm is determined at five positions arbitrarily selected, the average ratio is 80% or less. Patent Literature 5 describes a high-strength cold-rolled steel sheet having high chemical convertibility and containing, in terms of mass %, C: more than 0.1% and Si: 0.4% or more, in which the Si content (mass %)/Mn content (mass %) is 0.4 or more, the tensile strength is 700 MPa or more, the surface coverage ratio of Si-based oxides mainly composed of Si on the steel sheet surface is 20 area % or less, and the diameter of the maximum inscribed circle inscribing a region covered with the Si-based oxides is 5 μm or less. Patent Literature 6 describes a high-tensile strength steel sheet having high chemical convertibility containing, in terms of mass %, C: 0.01 to 0.30, Si: 0.2 to 3.00, Mn: 0.1 to 3.0%, and Al: 0.01 to 2.0% and having a tensile strength of 500 MPa or more, in which the average grain diameter of crystal grains on the steel sheet surface is 0.5 win or less; and when an observation region 10 μm or wider is sliced from the steel sheet surface to prepare a thin sample for cross-sectional TEM observation and the sliced thin sample is measured by TEM observation under conditions that enable observation of oxides 10 nm or smaller, the ratio of oxide species containing a total of 70 mass % or more of one or both of a silicon oxide and manganese silicate relative to the grain boundary region surface in the cross-section is 30% or less and the grain diameter of the oxide species present in a range of 0.1 to 1.0 μm in depth from the steel sheet surface is 0.1 μm or less.